Gas purge control for coolant in a fuel cell

ABSTRACT

A fuel cell includes a water transport plate providing a water flow field. The water flow field permits a flow of water having an entrained gas. A vent is in fluid communication with the water flow field. At least some of the gas is released from fuel cell by opening a vent. In a disclosed example, a valve is opened in response to conditions indicative of an undesired amount of gas. For example, the valve is actuated in response to a signal from a water level sensor. In another example, the valve is opened based upon a schedule.

BACKGROUND

This application generally relates to fuel cells, and more particularly,the application relates to managing gases within a fuel cell.

A fuel cell uses a cathode and anode that receive oxidant, such as air,and fuel, such as hydrogen, respectively, to generate an electrochemicalreaction that produces electricity, as is well known. Typically, thecathode and anode are separated by a solid separator plate whichprevents commingling of reactant gases but provides for electricalconductivity. The fuel cell typically includes numerous cells that forma stack. The cells may include water transport plates, which are porousseparator plates through which water passes, but not appreciablequantities of gas. The water transport plate is hydrated by a water flowfield on one side, the water flowing through the plate to humidify thereactant stream (fuel or oxidant) on the other side. The humidifiedreactant stream permits membrane hydration, which is important tosuccessful operation of the fuel cell. The water transport plate alsoenables removal of product water which is generated on the cathode bythe electrochemical reaction. In some example fuel cells, the circulatedwater acts as a coolant.

The volume of water within the stack must be managed to maintain adesired amount of water, for example, for membrane hydration, cellcooling, and minimizing the effects of sub-freezing environments. In onetype of cooling system, water is evaporated into a cathode reactant flowfield and then condensed in an external device to return liquid water tothe fuel cell's water flow field. Systems employing evaporatively cooledfuel cells have far less water than similar fuel cells using other typesof cooling strategies. However, gases may become entrained in thecoolant passages due to leakage from ambient surroundings, or reactantcrossover through the seals or the pores of the water transport plates,on the order of one cubic centimeter per minute per cell in the stack inone example. Entrained gases inhibit the replenishment of liquid waterto the water flow field, which can cause operational problems with thefuel cell. The gases must be expelled from the fuel cell to maintaindesired operation of the fuel cell.

What is needed is a method and apparatus of releasing gases from thecoolant passages of the fuel cell.

SUMMARY

A fuel cell includes a separator plate providing a coolant flow field.The coolant flow field receives condensed water from the cathodeexhaust. The coolant channels, which may be dead-ended, permit water topass through the anode water transport plate whereupon it humidifies themembrane and is subsequently evaporated into the cathode reactant streamto control the temperature of the fuel cell. The coolant flow field hasundesired entrained gas. A vent is in fluid communication with thecoolant flow field. The gas is released from the fuel cell by openingthe vent. The vent is opened in response to conditions indicative of anundesired amount of gas. In one example, a valve that is normally closedis actuated to open in response to a signal from a coolant level sensor.In another example, the vent is opened based upon a schedule.

Accordingly, gases can be released from the fuel cell to avoid gas buildup.

These and other features can be best understood from the followingspecification and drawings, the following of which is a briefdescription.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is schematic view of a fuel cell arrangement including anevaporative cooling loop.

FIG. 2 is a schematic view of a coolant flow field with a vent.

FIG. 3 is a schematic view of the vent shown in FIG. 2 with a controlvalve arranged in the vent and actuated in response to a level sensor torelease gas and retain coolant.

FIG. 4 is a schematic view of the vent shown in FIG. 2 with the controlvalve arranged in the vent and actuated in response to a controller torelease gas and retain coolant.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 schematically illustrates a fuel cell 10 that includes an anode12 and a cathode 14. The anode 12 receives fuel, such as hydrogen, froma fuel source 18. The cathode 14 receives an oxidant, such as air, froma source such as a blower 22. The oxidant chemically reacts with thefuel in an electrode assembly 16 that is arranged between the cathodeand anode 14, 12. The anode, cathode and electrode assembly 12, 14, 16provide a cell 11. Multiple cells 11 (only two shown) are arranged toprovide a stack. Electrically conductive separator plates 44 are used toseparate individual cells.

A separator plate 44 configured as a water transport plate comprises awater flow field 24 (FIG. 2) in fluid communication with the anode andcathode 12, 14 of each cell. In this example, at least a portion of thewater transport plate 44 for at least one of the cathode 14 or anode 12is porous. The water flow fields 24 are fluidly connected to one anotherby a coolant manifold 20 (shown schematically in FIGS. 3 and 4). Thewater flow fields may be dead-ended such that no liquid water iscirculated through the system, and the only movement of water is toreplenish that which has evaporated. Water 50 within the water flowfield 24 hydrates the water transport plates 44. An accumulator 26 isalso filled with water to ensure that the fuel cell 10 has a desiredvolume of water for desired operation of the fuel cell 10.

In another example, the water flow field is replaced by a coolant flowfield 24 wherein the coolant contains a percentage of water in a lowvapor pressure carrier, and the percentage of water is sufficient toevaporatively cool the cell.

In yet another example, one of the separator plates is solid. One sideof the solid separator plate has reactant flow fields; the other sidehas a coolant flow field allowing water to humidify the adjacentreactant flow field through the adjacent porous plate.

Water passes through the water transport plate 44, humidifies thereactant stream, and hydrates the membrane in the electrode assembly 16.Water formed by the electrochemical reaction on the cathode side of theelectrode assembly 16, as well as water passing through the membrane byosmotic drag, is evaporated into the cathode reactant stream of thecathode 14 on the opposite side of the water flow field 24. Oxidantpumped through the reactant flow field increases in temperature andbecomes saturated as it receives the evaporated water vapor. A cathodeexhaust loop 28 receives cathode exhaust (substantially depleted ofoxygen) with water vapor, and The water vapor is condensed with acondenser 30 and fan 32, or a similar arrangement. Liquid water 36 iscollected in a separator 34 and some of the gases are vented through anexit 40 in the separator 34. A return line 38 supplies the liquid water36 back to the water flow field 24 of the fuel cell 10.

Referring to FIG. 2, an example water transport plate 44 is shown havingchannels 46 that direct water through the coolant flow field 24. Gasbubbles migrate to a vent 42 in the coolant exit of manifold 20 (FIGS. 3and 4), which may have transparent portions for viewing the water level.The coolant manifold 20 communicates with the water transport plates 44associated with each cell in the fuel cell 10. The gases accumulateduring operation of the fuel cell 10 and must be frequently released tothe atmosphere.

Referring to FIG. 3, an example is shown in which a valve 54 is actuatedto release the gases 52 to atmosphere in response to a signal from alevel sensor 58. The valve 54 normally blocks a passage 53 incommunication with the coolant manifold 20. The closed position is shownin solid lines in FIGS. 3 and 4, and the open position is shown indashed lines. As the coolant level rises to a predetermined level, thelevel sensor 58 sends a signal to an actuator 56 to briefly open thevalve 54, which releases the gases that have collected in the coolantmanifold 20. In this manner, undesired gas build up is avoided.

Another example embodiment is shown in FIG. 4. FIG. 4 illustrates anarrangement in which the valve 54 is periodically opened based upon aschedule. A controller 60 contains information based upon one or morecharacteristics that are indicative of gas build up in the fuel cell 10.A schedule can be determined from these characteristics and used to openthe valve 54 using the actuator 56. In one example, fuel cell operatingtime is used to actuate the valve 54. In another example, the valve 54is opened at preset intervals.

Although several example embodiments have been disclosed, a worker ofordinary skill in this art would recognize that certain modificationswould come within the scope of the claims. For that reason, thefollowing claims should be studied to determine their true scope andcontent.

1. A fuel cell comprising: a plate providing a dead-ended coolant flowfield for permitting a flow of coolant having an entrained gas; and avent in fluid communication with the coolant flow field, the ventproviding a passage selectively opened and closed in response to asignal for releasing at least some of the gas.
 2. The fuel cellaccording to claim 1, wherein the plate is a water transport plate. 3.The fuel cell according to claim 1, wherein the plate is a solidseparator plate.
 4. The fuel cell according to claim 1, comprising acathode exhaust loop in fluid communication with the coolant flow fieldthat receives water vapor, the cathode exhaust loop including acondenser for condensing the water vapor to liquid water, a separatorfor separating the water vapor and liquid water from gases, and a returnline for supplying the liquid water to the coolant flow field.
 5. Thefuel cell according to claim 1, comprising a level sensor for detectinga coolant level, the level sensor providing the signal.
 6. The fuel cellaccording to claim 1, comprising a controller providing the signalaccording to a schedule.
 7. The fuel cell according to claim 6, whereinthe schedule is a time interval.
 8. The fuel cell according to claim 1,comprising a valve arranged in the vent to control when the vent isopened to release at least some of the gas.
 9. The fuel cell accordingto claim 8, wherein one side of the valve is exposed to atmosphere. 10.A method of venting gas entrained within fuel cell coolant comprisingthe steps of: blocking a passage having coolant that includes a gas; andselectively unblocking the passage in response to a signal to release atleast some of the gas.
 11. The method according to claim 10, comprisingthe step of evaporating water in the coolant to cool the fuel cell, andcondensing water vapor in the cathode exhaust to liquid water to returnthe liquid water to the fuel cell.
 12. The method according to claim 10,wherein the selectively unblocking step includes moving a valve from aclosed position to an open position.
 13. The method according to claim10, comprising the step of determining at least one characteristic ofthe fuel cell indicative of a need to open the vent to produce thesignal.
 14. The method according to claim 13, wherein the characteristicincludes coolant level.
 15. The method according to claim 13, whereinthe characteristic includes a schedule.
 16. The method according toclaim 15, wherein the schedule is based upon fuel cell operation. 17.The method according to claim 15, wherein the schedule is based upon atime interval between opening the vent.